One state-of-the-art method for obtaining the image of the internal structure of an object is widely known to comprise irradiation of the object under test with an X-ray beam and recording the radiation that has passed through the object with the aid of a detector, e.g., an X-ray sensitive film. Useful information appears, after decoding X-ray photographs, as data on the amount of radiation absorbed by the areas of the object under test having different degrees of radiotranslucency.
However, X-rays used in the heretofore-known method is a continuous X-ray spectrum polychromatic radiation. This offers some difficulties in interpretation of the results obtained, inasmuch as the long-wave spectral components are as a rule absorbed by the object itself and the proportion of absorbed radiation varies from one object to another, which cannot be allowed for at all times. This in turn results in adversed reliability of the results of inspection of an internal structure of the object under test.
One more state-of-the-art method for obtaining the image of the internal structure of an object is known to provide irradiation of the object under test with an X-ray beam collimated by a single-crystal monochromator, followed by recording the radiation passed through the object in a detector (W. A. Ellingson, M. W. Vanier "Application of Dual-energy X-ray Computed Tomography to Structural Ceramics", Adv., X-ray, Anal., 1988, v. 32, pp. 629-640).
The method mentioned above provides for higher sensitivity and reliability of the results of inspection due to the use of monochromatic radiation.
However, the method in questions is practically unsuitable, as the one discussed before, for testing the internal structure of objects featuring weak absorption of a penetrating radiation, especially such portions of the object under test, wherein the degree of X-ray transparency is close to that of the surrounding medium.
Finally, a method for obtaining the image of the internal structure of an object is also known, consisting in that the flux of a penetrating radiation emitted by a source is collimated by a first single-crystal monochromator, the object is irradiated with the collimated radiation, the radiation that has passed through the object is collimated by a second single-crystal and is then recorded by a detector (SU, A, 1,402,871).
According to the method under discussion, the radiation incident upon the object and passed therethrough is effected in an angular range corresponding to the characteristic angles of refraction of the radiation in question effective at the boundary of different-density media of the object under test. Used for collimation are perfect-quality single crystals placed in front of and past the object under test parallel to each other so that the Miller indexes of the reflecting surfaces thereof have the same value and be opposite as to sign.
With the above condition fulfilled the rays that have not refracted when passing through the object and did not deflected from the initial direction set by the first single-crystal monochromator, are reflected from the second single-crystal, which is recorded by the detector in a diffracted beam. At the same time the rays that have deflected from the initial direction at the boundary of two different-density media go beyond the limits of the range of Bragg reflection of the second single crystal so that a drop of the radiation intensity in the detector of the diffracted beam at said interface. Thus, the image of the internal structure of an object is established according to the method discussed above. The method is featured by a higher sensitivity compared with the aforediscussed methods for testing the internal structure of an object against the amount of absorbed radiation and is capable of testing such objects that are characteriezd by weak absorption of penetrating radiation.
However, when the object under test features an boundary between two media, wherein their refractive indices are so close to each other that the angle of deflection of rays at said boundary falls within the range of the angles of Bragg reflection of the second single crystal, such an boundary would not be registered by the detector because the rays deflected at said boundary get onto the detector receiving surface simultaneously with the rays that have been passed through the object without deflection. Thus, the deflected rays do not cause a perceptible reduction of the radiation intensity against the background of the non-deflected rays.